Several lines of evidence suggest that cell-mediated immune mechanisms are responsible for inducing latency in the course of syphilitic infection. The purpose of this investigation is to determine why activation of cell-mediated immunity (CMI) is not manifest earlier in syphilis. Data obtained to date suggest that the reaction of lymphocytes from syphilitic subjects is abnormal following in vitro exposure to Treponema refringens. These studies will now be repeated using Treponema pallidum the causative agent for syphilis.